Okay, time for another Brass Buckles random thought!
Previously I'd been thinking about the Garrus romance in terms of just 'Well maybe he liked Shepard and didn't realize it, until he realized how much he wanted to indulge in hot interspecies intercourse with her.' Then I thought more about the dialogue and the order it's in. My theory is now revised: I believe that when Shepard offers to "blow off steam" with Garrus, he interprets the situation as a romantic boyfriend/girlfriend relationship from the start, even though she presents the offer of sex in a casual context.
There's no evidence to point directly to this, but it seems to me, given Garrus's dialogue, his main concern may never have been how they're going to manage the interspecies sex, or blowing off steam at all. No, because when Shepard offers to help him with that de-stressing he needs to do so badly, I don't think it would matter much to him whether or not he respected her deeply or that she's one of very few friends he has left if he thought it was purely about the sex. I don't think he had much respect for that recon scout, and he was perfectly willing to have sex with her. He's voicing his reasons why he'd agree to pursue a romantic relationship with Shepard, or why it should be more than just sex, not why he'd have casual sex with her. He may even be thinking that it's potentially long-term when he puts so much emphasis on their friendship and his respect for her. Add to this the fact that surely he knows that fraternizing with his superior officer is bad and that the human/turian thing is dangerous. He might like risk, but he doesn't usually take risks unless they mean something to him.
The very next real conversation with Garrus involves his thinking they're crazy, but then he asks if Shepard would rather be with somebody less exotic. That's not a matter of him trying to get out of a dangerous situation. It might be crazy, but he wants to be with Shepard. Only when she tells him "I don't want somebody closer to home. I want you. Someone I know. Someone I can trust," does he reply, "I can do that." It'd be an odd line for him to choose, except that he's saying he wants to be that familiar, trusted person she can lean on. It's not about Garrus being nervous about the sex or how dangerous it might be, or saying that sure, he can have sex with her as long as she wouldn't rather have a human or drell or asari. It's more likely that he's nervous that sex is all Shepard wants (especially since it may go badly or, for all he knows, it may not even be possible--although he obviously hopes it'll go really well). Here she is, the person he respects more than anybody else, and she wants to have sex with him. It would be a letdown if she were just using him, especially if the sex were to go badly, in which case he'd probably lose face in the eyes of this person whom he greatly respects. He's reassured only if she expresses that it's because she actually likes Garrus as a person. Garrus also makes it plain that he's not physically attracted to humans, which means that the sex alone isn't the kind of benefit it would be for a human male who got the same offer.
Once he's reassured that interspecies intercourse is not all Shepard is after, Garrus claims he wants to delay the sex until the last possible moment to avoid disrupting the crew more than necessary. I take this to mean several things--one, that he has no intention of hiding what they're doing, two, that he's aware the crew will probably notice to some degree anyway, and three, it's not really just the sex he's after (if that's what he wants at all) or he might shrug off concerns about the crew or Shepard's reputation to have as much xenophilic sex as possible. Though he makes a metaphor that completely undermines what he means, "savoring the last shot" probably refers to enjoying as much time as he can with Shepard before they finally throw caution to the wind and consummate their new romantic relationship.
While I now think it's meant more to show that Garrus thinks Shepard is offering a romantic relationship from the beginning, I still think that there must have at least been some subconscious attraction from the beginning (not secret love, but maybe an unacknowledged crush). If he then believes she wants a romantic relationship, and realizes that he's attracted to her enough to do so, then his reaction when the romance is ended becomes more believable. The more reason he has to believe that this new romantic aspect to their relationship is really going to work out, and that he's not imagining that Shepard is interested in him as a person, the more upset he is when she tells him otherwise, right up until she might stomp on his still-beating heart with her armored boot heels when he shows up before the suicide mission wearing a nice suit and bringing a bottle of wine (likely wine he can't even drink). And that isn't to say he can't get over her and find someone else, but dumping him any time after telling him that she wants Garrus--someone she can trust--would mean Shepard was stringing him along that whole time.
But enough of my random rambling. There are way too many interpretations possible and I only just came to think of this one as making more sense (to me) than most others. I kind of felt the need to share because it struck me as an "aha!" moment, given that it makes all of the Garrus scenes a little more romantic than they were before: Whether Shepard knows it or not, Garrus may be looking at this as potentially long-term from the moment that she brings up the option. Let's face it, except for that last scene, Garrus's romance isn't really all that romantic otherwise. Well, okay, I'll give you the "few moments that are just for us" line, but mostly he's just seemingly worried about how they're going to have sex. If you think that maybe Garrus had wanted a deeper connection (romantically or not) to Shepard all along from the start of the romance (and possibly before), that he's nervous that he's going to ruin that aspect of it with really bad sex (or inability to have sex with her at all) or that Shepard doesn't feel the same way, then all of the scenes become a lot sweeter.